returns the default color for the nth data series.
This may be a system wide color or a color coming from a
color 計画／陰謀.
Usually there 存在する a 直す／買収する，八百長をするd number of default colors. This
method should always return a valid Color. If the 索引 (i)
is higher than the number of default colors (n), the method
should return the modulus (i mod n), i.e., the colors should
repeat in a cyclic way.
Parameter nIndex
The 索引 of the series. This is used to 得る the
訂正する default color.
